Pick and place machines are generally used to manufacture electronic circuit boards. A blank printed circuit board is usually supplied to the pick and place machine, which then picks electronic components from component feeders, and places such components upon the board. The components are held upon the board temporarily by solder paste, or adhesive until a subsequent step in which the solder paste is melted, or the adhesive is fully cured.
Pick and place machine operation is challenging. Since machine speed corresponds with throughput, the faster the pick and place machine runs, the less costly the manufactured board. Additionally, placement accuracy is extremely important. Many electrical components, such as chip capacitors and chip resistors are relatively small and must be accurately placed on equally small placement locations. Other components, while larger, have a significant number of leads or conductors that are spaced from one another at a relatively fine pitch. Such components must also be accurately placed to ensure that each lead is placed upon the proper pad. Thus, not only must the machine operate extremely fast, but it must also place components extremely accurately.
In order to enhance the quality of board manufacture, fully or partially populated boards are generally inspected after the placement operation(s), both before and after solder reflow, in order to identify components that are improperly placed or missing or any of a variety of errors that may occur. Automatic systems that perform such operation(s) are highly useful in that they help identify component placement problems prior to solder reflow allowing substantially easier rework or identify defective boards after reflow that are candidates for rework. One example of such a system is sold under the trade designation Model KS 200 available from CyberOptics Corporation of Golden Valley, Minn. This system can be used to identify such problems as alignment and rotation errors; missing and flipped components; billboards; tombstones; component defects; incorrect polarity; and wrong components. Identification of errors pre-reflow provides a number of advantages. Rework is easier; closed-loop manufacturing control is facilitated; and less work in-process exists between error generation and remedy. While such systems provide highly useful inspection, they do consume plant floor-space as well as programming time maintenance efforts and the like.
One relatively recent attempt to provide the benefits of after-placement inspection located within a pick a place machine itself is disclosed in U.S. Pat. No. 6,317,972 to Asai et al. That reference reports a method for mounting electric components where an image of a mounting location is obtained prior to component placement, and compared with an image of the mounting location after component placement to inspect the placement operation at the component level.
While the disclosure of Asai et al. marks one attempt to employ in-machine component level inspection, there remains much work to be done. For example, the disclosure of Asai et al. is primarily related to turret-style pick and place machines, where the placement position does not move in the x and y directions, but simply moves up and down. In such systems, relatively large and heavy imaging systems can be provided proximate the nozzle(s), image a plurality of placement events and still have little, or no, adverse impact on placement machine speed or design layout. In contrast, on gantry-style pick and place machines (given relatively little attention by Asai et al.) the nozzle moves in at least one of the x and y directions. Thus, optics intended to image a plurality of placement events also move in x and/or y. Accordingly, the size and mass (inertial load) of the optical system itself can be prohibitive of on-head use in gantry-style machines. Further, since the head of a gantry-style pick and place machine is moving in x and/or y, it is important to minimize the size of the optical system to reduce the possibility that it will collide with other portions of the pick and place machine.
For pick and place machines having heads that move in x and/or y, increased mass is an issue because of the increased inertia. Achieving a certain machine throughput is dependent, in part, on the head's acceleration. Given a certain motive power provided by the electromechanical system of the pick and place machine, increased mass causes decreased acceleration.
Size, that is volume and/or shape of the optical system attached to the moving head, can also be a problem for a number of reasons. One reason is that the head may be designed so as to just fit in its environment as it moves about its workspace without colliding with anything. Adding something that protrudes beyond the spatial envelope of the existing head structure must be done with care and consideration of the possibility of physical conflict. Another reason that the size and/or shape of the head can become a problem is that there are generally a relatively large number of cables, tubes, motors, and other structures mounted to the head. Adding something that may conflict with assembly or maintenance of the machine is generally disfavored.
To increase the viability of component level placement inspection in a pick and place machine, it would be advantageous to improve aspects of the optical system, the illumination system, image acquisition, and image processing without adversely affecting machine throughput or design. Further, it would be advantageous to provide optical systems and techniques that are practicable in both turret-style and gantry-style pick and place machines.